The basic principles and implementations of cold cathode discharge lamps (commonly and hereafter referred to as neon lights) are relatively well known. However, several technical challenges have prevented the widespread use of neon lights for rear lighting and signaling applications in automotive applications.
When designing a gas discharge lighting system particularly for automotive use, there are three very important issues to consider. One is the life expectancy of the lamp, the second is the lamps efficiency, and finally the optical performance must be considered.
The life expectancy of a gas discharge lamp is defined by three main factors, the gas mass present which obviously is affected by the chosen fill pressure and volume, the operating current, and the surface area of the electrodes. It is well known within the industry that aging process of a neon lamp occurs when molecules of the gas are absorbed within the electrodes. The rate of absorption increases rapidly as current increases, which then causes the electrode to "sputter" onto itself and the glass walls of the discharge tube. The deposited material will then trap more gas molecules. Once this process has started, it will likely continue and accelerate until there will be insufficient gas pressure to maintain a discharge and the lamp will eventually fail. This process is the major factor in determining the lamps life and is commonly called gas clean-up.
In the areas of lamp efficiency, it is very important to consider the voltage drop of the discharge itself. This is dependent on two very important parameters, the cross sectional area of the discharge path and the gas pressure.
To generate a light output, electrical energy is transferred to a volume of encapsulated gas, changing the energy state of the gas molecules. As the gas molecules go to a lower energy state, light is emitted. If a discharge path is constrained within a smaller cross sectional area, a higher portion of the energy used in exciting the gas will be released back as light relative to a relativity unconstrained discharge path. However, the constriction raises the resistance of the discharge, which in turn increases the voltage drop, which must be considered when designing the lamp. In addition, reducing the cross sectional area also lessens the gas volume which will then affect the lamp life.
The other key parameter when considering lamp efficiency is gas pressure. In general, lowering the gas pressure increases the efficiency. In a gas at a lower pressure there are fewer molecules per cubic cm than in a gas at higher pressure. Therefore, in the low pressure gas, free electrons have a longer distance in which to gain speed before they hit a neutral molecule. The consequence of this is greater molecule collisions which in turn generate more light. There is a point however when the pressure is too low and the efficiency again falls due to a lack of charge carriers within the discharge tube. It is also important to note, that when very small cross sectional diameter tubes are used to take advantage of the light gains as mentioned above, the small gas volumes available in a low pressure lamp will result in a relatively short lamp life.
The final area to consider when designing a lamp for automotive use is the optical performance of the entire lamp package. A neon tube emits light evenly around its circumference As with any light source, in order to achieve the highest efficiency, the light must be collected and directed in the appropriate direction for each particular application. In addition particular applications may require certain colors or wavelengths. Commonly, this has been achieved by filtering light through certain colored lenses.
U.S. Pat. No. 5,608,288 (hereinafter Haag), also assigned to the assignee of the present invention, provides a neon lamp suitable for automotive application which improves lighting efficiency while providing a design which improves operational life. Haag provides an automotive neon lamp having a first semicircular reflector surface joined to a semi-parabolic surface. Additionally, the neon lamp of Haag has a volumetric concentrator which aids in maintaining the position of the electron discharge path of the neon lamp while at the same time allowing for a greater volume of neon, thereby increasing lamp life while retaining better properties of focusability.